The present invention relates to a novel agent for blocking polyisocyanates. The present invention also relates to novel blocked polyisocyanates produced with the aid of the novel agent. Last but not least, the invention relates to novel coating compositions comprising the novel blocked polyisocyanates.
Here and below, the term "blocking" denotes the reversible reaction of isocyanate groups with appropriate compounds, such as hydroxyfunctional compounds, for example, to form groups which when heated release the blocking agent and the isocyanate groups. The latter are then available again for crosslinking reactions in the coating compositions. The reversal of blocking is also widely known as deblocking.
On grounds of energy, cost and a process regime which does not subject the object to be coated--a car body, for instance--to excessive thermal loading the aim is to use fully blocked polyisocyanates which effect complete crosslinking of the coating compositions at comparatively low temperatures, i.e., at or below 150.degree. C., and in a comparatively short time, i.e., during 10 to 30 minutes.
Patents EP-A-0 206 071 and U.S. Pat. No. 5,086,090 disclose for this purpose blocking the polyisocyanates with furfuryl alcohol. However, the polyisocyanates blocked with this agent have an extremely great tendency to crystallize, which in electrodeposition coating materials in particular is manifested to disadvantage by the formation of unwanted precipitates after just a short time of standing. This occurs in particular with fully blocked diphenylmethane diisocyanates, which are used with preference in electrodeposition coating materials on account of their advantageous technological properties: in particular, their yellowing stability.
The disadvantageous great tendency to crystallize can be countered to a certain degree by subjecting the polyisocyanates to only partial blocking with furfuryl alcohol in a first stage before carrying out full blocking of the resultant partially blocked polyisocyanate with trimethylolpropane in a second stage. A disadvantage of this conventional process is the additional step, which requires additional expenditure on monitoring, time and energy. Furthermore, trimethylolpropane is a solid and must be added either as a melt or as a powder, either option being comparatively laborious.
It is an object of the present invention to provide a novel, furfuryl alcohol-based blocking agent which permits the simple, elegant and full blocking of polyisocyanates in one stage and results in novel fully blocked polyisocyanates having a very small tendency to crystallize and a comparatively low deblocking temperature. A further object of the present invention is to provide novel coating compositions, especially electrodeposition coating materials, which no longer have the disadvantages of the prior art but instead exhibit no unwanted precipitation even on long times of standing, lend themselves to thermal crosslinking at comparatively low temperatures, and result in coatings having an outstanding profile of performance properties and, in particular, no yellowing tendency.